In a large skillet, melt the butter and olive oil together over medium-high heat. Add the mushrooms, and cook until they begin to soften, about 3 to 4 minutes. Add the leeks and garlic, and cook until tender, about 3 minutes. Add the broth, cream, lemon juice, and salt; cook for 2 minutes. Stir in the spinach, and lower the heat, stirring frequently, until the spinach is wilted, about 2 minutes. Sprinkle in the cheese and stir to combine. Transfer to a serving bowl and serve immediately.